There have been recent efforts to develop drilling techniques that do not require physically cutting and scraping material to form the borehole. Particularly relevant to the present disclosure are pulsed electric drilling systems that employ high energy sparks to pulverize the formation material and thereby enable it to be cleared from the path of the drilling assembly. Such systems are at illustratively disclosed in: U.S. Pat. No. 4,741,405, titled “Focused Shock Spark Discharge Drill Using Multiple Electrodes” by Moeny and Small; and WO 2008/003092, titled “Portable and directional electrocrushing bit” by Moeny; and WO 2010/027866, titled “Pulsed electric rock drilling apparatus with non-rotating bit and directional control” by Moeny. Each of these references is hereby incorporated herein by reference.
Generally speaking, the disclosed drilling systems employ a bit having multiple electrodes immersed in a highly resistive drilling fluid in a borehole. The systems generate multiple sparks per second using a specified excitation current profile that causes a transient spark to form and arc through the most conducting portion of the borehole floor. The arc causes that portion of the borehole floor penetrated by the arc to disintegrate or fragment and be swept away by the flow of drilling fluid. As the most conductive portions of the borehole floor are removed, subsequent sparks naturally seek the next most conductive paths. If this most conductive path is created by the residue of the previous disintegration, the subsequent sparks will be shunted through the residue rather than through the formation, negating the intended effect of the drilling process. The known pulsed-electric drilling systems and methods do not appear to adequately address this issue.
It should be understood, however, that the specific embodiments given in the drawings and detailed description do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed in the scope of the appended claims.